The present invention relates to devices for attenuating noise of turbine engines. The invention relates more particularly to an acoustic muffler for an exhaust of a turbine engine, the muffler defining a plurality of resonating chambers.
Turbine engines are often used in applications in which personnel must work in relatively close proximity to an engine while it is operating. Examples of such applications include use of turbine engines as auxiliary power units (APUs) in various settings, such as on aircraft or watercraft. The noise generated by the engine, if too great, could be detrimental to the personnel. Accordingly, it is not uncommon for noise suppression devices to be required to reduce the noise of the engine to an acceptable level.
One type of noise suppression device is an acoustic muffler that is mounted in the exhaust duct of the engine to reduce the noise associated with the stream of exhaust gases discharged from the engine. A conventional muffler consists of a cylindrical or conical outer housing or shell located concentrically with respect to the exhaust duct and lined with a cylindrical or conical porous sheet metal liner concentric with the outer shell and of smaller diameter than the shell so as to create an annular chamber between the liner and the shell. The chamber functions as a resonator. In the case of a cylindrical muffler, the chamber is tuned to attenuate noise at a single frequency, whereas the chamber of a conical muffler will attenuate noise over a range of frequencies. The porous liner provides broad band noise attenuation.
The exhaust gases from the engine are at a very high temperature. Care must therefore be taken to provide adequate separation between the muffler and other structures in proximity to the muffler. For example, in the case of an APU mounted in the tail cone of an aircraft, the engine exhaust duct typically runs close to the airframe structure. In many aircraft, the airframe structure desirably is constructed of aluminum alloy, as opposed to special high-temperature materials such as titanium alloy or steel, because aluminum is less expensive and easier to process than such materials. However, with the above-described conventional muffler, in some cases it may be impossible to provide sufficient separation between the muffler and the adjacent structure to enable aluminum to be used for such structure. In this event, the airframe structure must be constructed from special high-temperature materials.
It would be desirable, therefore, to provide an acoustic muffler that enables an increased physical separation between the muffler and adjacent structure without sacrificing noise attenuation performance.
The present invention addresses the above needs by providing an acoustic muffler in which the outer shell and the inner liner are eccentrically located with respect to each other so that when the liner is mounted concentric with respect to the exhaust duct of the engine, the outer shell on one side of the duct projects radially outwardly a smaller distance than would that of a comparable conventional muffler. This side of the outer shell can be oriented toward the closest adjacent structure, thus providing greater separation between the muffler and the structure. The acoustic muffler also provides both broad band frequency attenuation and specific attenuation at a plurality of tuned frequencies.
More particularly, in accordance with a preferred embodiment of the invention, the acoustic muffler comprises an outer shell of tubular configuration, a tubular liner disposed within the shell, and a plurality of dividing walls extending between the liner and the outer shell so as to divide the generally annular space between the liner and shell into a plurality of chambers circumferentially spaced about the muffler. A central longitudinal axis of the liner is radially offset from the central longitudinal axis of the shell such that the chambers have different depths in a radial direction of the muffler, and the liner has at least one opening into each chamber. Thus, the muffler has a plurality of chambers of different sizes providing noise attenuation at a plurality of specific tuned frequencies, and the liner provides broad band attenuation.
In a particular cylindrical embodiment of the muffler, a middle portion of the side wall of the shell is formed as a substantially cylindrical structure, and the side wall includes forward and aft portions respectively joined to forward and aft ends of the middle portion. The forward and aft portions each extends radially inwardly from the middle portion to a radially inner end, and the radially inner ends of the forward and aft portions are joined to the liner. Each chamber extends longitudinally from the forward portion of the side wall to the aft portion of the side wall.
Alternatively, the shell and liner could be conical or could have other than a circular cross section. One or more of the chambers could be subdivided by partition walls into two or more chambers located one after another in the longitudinal direction of the muffler.
Various numbers of chambers can be provided depending on the specific frequencies at which attenuation is desired. In a particular embodiment of the muffler designed for installation in the exhaust duct of a Honeywell 331-400B auxiliary power unit in the tail cone of a 767-400ER aircraft, there are six chambers. The chambers all have approximately the same length in the circumferential direction. The radial depth of the chambers varies continuously in the circumferential direction from a minimum at one side of the muffler to a maximum at the opposite side of the muffler. The depth of each chamber in the radial direction is substantially constant from the forward to the aft end of the chamber. The deepest chamber is slightly more than three times as deep as the shallowest chamber. The side of the muffler having the shallowest chamber is oriented toward the tail cone frame and skin closest to the muffler. The muffler allows sufficient separation between the muffler and the frame and skin such that there is no need to use a high-temperature material for these structures. Furthermore, the muffler reduces the noise at the critical service location by about 3 dBa relative to an exhaust duct without the muffler.
It will be recognized, however, that the principles of the present invention can be applied to various types of turbine engine installations in which the optimum acoustic performance of the muffler and the physical constraints of the installation may dictate a different number of chambers, a different range of chamber depths, etc. The muffler configuration thus is specific to the installation.